Such a circuit arrangement is known from the printed publication “Electric Amplifier Module for Controlling Directly Controlled Regulating Valves having an Electric Feedback of Type VT 11080, Series 2X” (RD 29 757/04.93) of the Mannesmann Rexroth GmbH. The circuit arrangement has an amplifier circuit having an input stage and an output stage. The input stage is supplied with an electrical input signal, e.g. in the form of a direct voltage variable between 0 and 10 V. This voltage is in the most simple case a voltage tapped off from a potentiometer. Alternatively, this may be a voltage supplied by a programmable controller normally referred to as “SPS” or a voltage supplied by another primary controller. The voltage supplied to the input stage is used as a setpoint value for the current that is to be supplied to the solenoid coil. If required, other signals may be added in the input stage to form a control signal for the output stage. The output stage converts the control signal into a current. This current is supplied to the solenoid coil via connecting lines situated between the output stage and the solenoid coil. For this purpose, e.g. a range of the current between 0 and 1 A corresponds to a range of the input voltage from 0 to 10 V.
In order to satisfy the safety requirements of the ATEX regulations, it is necessary, among other things, to ensure that the surface temperature of the individual components of a system is always lower than the ignition temperature of the gas surrounding the components. In the case of electrically controlled fluid valves having a control element operated by a solenoid coil, the solenoid coil is a component that heats up in operation as a function of the current. What makes things more difficult in this regard is the fact that the boundary values for the current flowing across the solenoid coil in many cases lies just barely above the value required for the full deflection of the control element of the fluid valve. This means that the safety margin between the current required for the full deflection of the control element of the fluid valve and the boundary value of the current specified by the construction of the solenoid coil is in the order of only a few percent, e.g. 5% of the greatest operationally intended current. In normal operation, this boundary value is not reached; but a series of cases are conceivable, in which the boundary value of the current flowing across the solenoid coil is reached and even exceeded. This includes e.g. an overloading of the amplifier input in which the input voltage supplied to the input stage of the amplifier is greater than the input voltage associated with the greatest operationally intended current. Another case, in which a current exceeding the boundary value may occur is a faulty operation of the parameter setting of the amplifier on the part of the customer, which results in an output current that exceeds the greatest operationally intended current. Another conceivable case, in which a current exceeds the boundary value, may occur in the event of a short circuit between a line conducting an operating voltage and a connecting line leading from the amplifier to the solenoid coil. Printed publication RD 29 757/04.93 does not specify measures to prevent an unacceptably high heating of the solenoid coil in such cases.
From German Published Patent Application No. 195 15 640, a circuit arrangement is known for electrically controlling a solenoid-operated fluid valve. A controlled switch in the form of a contact of a contactor relay is situated in the connecting lines between an amplifier and a solenoid coil. This controlled switch is used to satisfy increased safety-related requirements such as must be met e.g. when using electrically operated hydraulic valves for controlling the flow of the pressure medium to cylinders that move the tools of a molding press. In a hazardous operating state it must be possible to shut down the machine in a safe manner. This is to occur e.g. if an end switch responds as the upper dead center of the press is reached, if an emergency switch has been operated or if a primary controller outputs an appropriate signal. In these cases, an enable signal is removed, which, in addition to the other functions it performs, also opens the controlled switch. If the valve is configured as a proportional valve having positive overlap and mechanical centering of the control piston, then, when no current is supplied to the solenoid coil, the control piston assumes a safe center position in which no pressure medium flows to or from the cylinder. Opening the controlled switch thus ensures that no current flows across the solenoid coil even in the case of an electrical fault of the amplifier. The circuit arrangement known from German Published Patent Application No. 195 15 640 does not address measures for preventing an unacceptably high heating of the solenoid coil.
A device for switching an electrohydraulic solenoid-operated directional control valve is known from German Published Patent Application No 24 26 512. Operating in normal operations as a switching transistor, a transistor, in accordance with a clocked control current that is supplied to its base terminal, connects in one switching state a solenoid coil to a supply voltage, while interrupting this connection in the other switching state. The magnitude of the current flowing across the solenoid coil is determined by the ohmic resistance of the solenoid coil and the magnitude of the supply voltage. For protecting the switching transistor, a protective circuit is provided, which opens a contact of a relay situated between the supply voltage source and the solenoid coil whenever a higher current than the normal operating current flows. In the case of a current that is only slightly greater than the normal operating current, the relay opens the contact between the supply voltage source and the solenoid coil. Afterward, the current flow remains interrupted due to a self-holding mechanism in the relay. In the case of a current that is significantly greater than the normal operating current, the control current of the switching transistor is additionally reduced in the time interval until the response of the relay. This circuit arrangement relates to an output stage, which applies a clocked voltage to a solenoid coil. No input stage is provided which converts a variable input voltage via a current controller or current regulator into a current corresponding to the magnitude of the input voltage such as e.g. in the circuit arrangement known from the printed publication RD 29 757/04.93 mentioned at the beginning. The problem of a limitation of the surface temperature of the solenoid coil is also not addressed.